1. Field of the Invention
The present invention relates to X-ray screening systems for airport luggage and the like; and, more particularly to screening systems that utilize radiation transmitted through and scattered from an object under inspection to detect weapons, narcotics, explosives or other contraband.
2. Description of the Prior Art
X-ray inspection systems that use transmitted radiation have conventionally been employed to detect the shape of high-Z material (Z refers to atomic number) such as steel. The principal objective of these systems is the detection of weapons, such as guns, knives, bombs and the like. A major problem with X-ray inspection systems is the inability thereof to accurately recognize and detect explosives and narcotics made up of low-Z materials. Recently dual energy X-ray systems have been used to improve the detection of low-Z material. Such systems measure the different attenuation that high and low transmitted energy X-rays experience as a result of passage through any material. This principle has allowed the identification of virtually any material so long as the material is not covered by a different Z material. In order to overcome the material overlaying problem, it has been proposed that X-ray transmission be effected from different directions using two X-ray sources, or that the object be scanned from all sides and the results be evaluated with computer tomography.
Another approach for identifying low-Z material involves detecting the Compton scattered radiation along with the transmitted radiation. Low-Z material such as explosives and narcotics generates more scattered radiation than high-Z material like iron. This scattered radiation differential provides a basis for distinguishing between low-Z and high-Z material in instances where the low-Z material is concealed behind high-Z material.
Among the more troublesome problems with X-ray transmission and Compton scatter images are their poor resolution and high noise content. The causes of these problems are traced to: a) the relatively poor light collection method used in converting X-ray photons to light photons; and b) photon integration. Poor light collection presents a problem because it requires use of slow (long persistence) phosphor type X-ray detectors. Such detectors oftentimes create blurred images owing to the slow response time of the excited phosphor. The use of photon integration in conventional signal generation produces noisy images, particularly in cases where the transmitted or backscattered X-ray rates are relatively small. For example, U.S. Pat. No. 5,260,982 to Fujii et al. discloses a scattered radiation imaging apparatus. The Fujii et al. apparatus employs long persistence phosphor type X-ray detectors and photon integration yielding relatively low resolution, high noise images.
Thus there remains a need in the art for a system and a method of x-ray inspection that would afford more rapid scanning and improved image quality, including higher resolution, reduced noise, and sharper definition of the image. Such improvements would permit items of interest, such as bombs, firearms and other weapons, currency, narcotics, incendiary materials, agents that pose biological, chemical, or radiological danger to people or property, and other contraband shapes and accompaniments to be recognized. Better systems are highly sought, especially in the context of airport screening, but would be equally valuable for courthouses, stadiums, schools, government offices, military installations, correctional institutions, and other public venues that might be targets of terrorist or similar criminal activity.